<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        //声明一个字符串
        let str = 'JS5211314你知道么555啦啦啦'
        //正向断言
        // const reg = /\d+(?=啦)/;
        // const result = reg.exec(str)
        // console.log(result);

        //反向断言
        const reg = /(?<=么)\d+/
        const result = reg.exec(str)
        console.log(result);
    </script>
</body>
</html>